I'm your guy.
We had an x17z that didn't have outdoor speakers and I LOVE having the same music playing inside the camper and outside the camper. I actually used to own a mobile entertainment company and I installed all my CD's on a laptop but have my favorite tunes on an old iphone. Here's what I do.
1) The first piece of the puzzle if the audio source. Say you have an ipad/iphone/ipod with the headphone jack that has the music you want to hear.
2) The next piece of the puzzle is an FM transmitter. Here's the one I have. FM Transmitter
You simply plug the ipod into the FM transmitter.
3) Before playing any music, turn on the radio in your camper and find an FM station that is complete static. Then turn on your FM transmitter and set it to broadcast the station that had all the static.
4) Tune your outdoor radio to the same station as the inside radio.
5) Open a cold beer and enjoy the tunes.
Even though we have indoor and outdoor speakers in our campers I will still use the FM transmitter. If I just plug my iphone into the camper radio then I have to blast the volume in order to hear the tunes on the outdoor speakers. Of course, this means the indoor speakers are screaming which isn't cool.
With the FM transmitter it gives you a greater flexibility in volume control. You can simply turn the camper;s indoor speakers at a comfortable level and then use the portable radio outside and set THAT to a comfortable level.
Does that make sense?